Fragmentation point and simulation site adjustment for resolution enhancement techniques

ABSTRACT

A method of performing a resolution enhancement technique such as OPC on an initial layout description involves fragmenting a polygon that represents a feature to be created into a number of edge fragments. One or more of the edge fragments is assigned an initial simulation site at which the image intensity is calculated. Upon calculation of the image intensity, the position and/or number of initial simulation sites is varied. New calculations are made of the image intensity with the revised placement or number of simulation sites in order to calculate an OPC correction for the edge fragment. In other embodiments, fragmentation of a polygon is adjusted based on the image intensities calculated at the simulation sites. In one embodiment, the image intensity gradient vector calculated at the initial simulation sites is used to adjust the simulation sites and/or fragmentation of the polygon.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. patent application Ser. No. 15/174,879, filed Jun. 6, 2016, entitled “FRAGMENTATION POINT AND SIMULATION SITE ADJUSTMENT FOR RESOLUTION ENHANCEMENT TECHNIQUES,” which is a divisional of U.S. patent application Ser. No. 14/059,183, filed Oct. 21, 2013, entitled “FRAGMENTATION POINT AND SIMULATION SITE ADJUSTMENT FOR RESOLUTION ENHANCEMENT TECHNIQUES” (now U.S. Pat. No. 9,361,422), which is a divisional of U.S. patent application Ser. No. 12/972,097, filed Dec. 17, 2010, entitled “FRAGMENTATION POINT AND SIMULATION SITE ADJUSTMENT FOR RESOLUTION ENHANCEMENT TECHNIQUES” (now U.S. Pat. No. 8,566,753), which is a continuation of U.S. patent application Ser. No. 11/067,504, filed Feb. 25, 2005, entitled “FRAGMENTATION POINT AND SIMULATION SITE ADJUSTMENT FOR RESOLUTION ENHANCEMENT TECHNIQUES” (now U.S. Pat. No. 7,861,207), which claims the benefit of U.S. Provisional Patent Application Nos. 60/564,138, filed Apr. 21, 2004, entitled “METHOD FOR DYNAMICALLY ADJUSTING SITES FOR USE WITH OPC USING GRADIENTS OF AERIAL IMAGE,” and 60/547,484, filed Feb. 25, 2004, entitled “CONCEPTS IN OPTICAL AND PROCESS CORRECTION.” U.S. patent application Nos. 15/174,879, 14/059,183, 12/972,097, and 11/067,504; and U.S. Provisional Patent Application Nos. 60/564,138 and 60/547,484 are all incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to photolithographic processing in general, and in particular to layout correction for resolution enhancement techniques such as optical and process correction (OPC).

BACKGROUND OF THE INVENTION

In the conventional photolithographic processing of integrated circuits, features are created on a semiconductor wafer by exposing the wafer with light or radiation that is passed through a mask or reticle. A typical mask/reticle has patterns of opaque and clear areas that selectively expose corresponding areas of light-sensitive chemicals on the wafer. The exposed areas are chemically and mechanically processed to create the desired features on the wafer.

As the size of features being created on a wafer approaches and becomes smaller than the wavelength of radiation used to expose the wafer, optical distortions can occur whereby the pattern defined on the mask or reticle will not match the pattern of features that are created on the wafer. To improve the pattern fidelity, changes can be made to the mask/reticle patterns that compensate for the expected optical distortions. One common tool for adjusting the mask/reticle pattern is an optical and process correction (OPC) tool such as the CALIBRE® software tools available from Mentor Graphics Corporation, the assignee of the present invention.

As will be appreciated by those skilled in the art, an OPC tool works to produce a corrected mask/reticle by reading at least a portion of a layout design that is defined in a database. Each feature to be created on the wafer is defined as a series of vertices that make up a polygon having a shape of the desired feature. The polygons are fragmented by dividing the perimeter of the polygon into a plurality of edge fragments. An edge placement error (EPE) is computed for each edge fragment that compares where an edge fragment will be printed on a wafer versus its desired position. The OPC tool then moves the edge fragments in order to precompensate for the expected optical distortions that will occur during processing so that the position of the edges created on a wafer will more closely match the desired positions.

FIG. 1A illustrates a representative polygon 10 that defines a rectangular feature to be created on a wafer. In order to correct for optical distortions, the polygon 10 is divided into a plurality of edge fragments that are bounded by fragmentation end points 12. During OPC, at least some of the edge fragments positioned between the fragmentation end points 12 are moved inwardly or outwardly to compensate for optical distortions. In the example shown in FIG. 1A, the polygon 10 does not contain a sufficient number of fragmentation points 12 to create the rectangular feature on the wafer with an acceptable image fidelity. A simulated aerial image 14 plots where the edge fragments will be printed on a wafer. In the example shown, the fragmentation of the polygon 10 is too coarse in order to be able to finely correct for the optical distortions that may occur during processing. Conversely, FIG. 1C illustrates a polygon 10 including more than enough fragmentation end points 12 to finely adjust for the optical distortions that may occur during processing. Although the number of fragmentation end points 12 is sufficient in the example shown in FIG. 1C, the time required to compute the OPC corrections of each individual edge fragment may be prohibitive. Therefore, it is desirable to divide the polygon 10 in a manner as shown in FIG. 1B with a sufficient number of fragmentation end points 12 so that image fidelity is acceptable and processing time is not prohibitive.

Associated with each edge fragment is a simulation site that defines a number of sample points at which the image intensity during photolithographic processing is estimated. From the estimated image intensity points, a determination is made of the expected edge placement error (EPE) of the edge fragment. FIG. 2 shows a conventional, simplistic method of placing the simulation sites on the edge fragments. Simulation sites 16 a are placed in the center of the edge fragments that are at the ends of the polygon and simulation sites 16 b are positioned at the location of the fragmentation end points 12 that are adjacent to the corners of the polygon. Additional simulation sites 16 c are placed in the center of the edge fragments that are between the fragmentation end points 12 for the remainder of the polygon. Comparing the location of the simulation sites with simulated aerial image 18 (which is a plot of the estimated image intensity at a value that will expose the chemicals on the wafer), it can be seen that many simulation sites are not positioned at the place where the aerial image intensity deviates most significantly from the desired outline of the polygon 10. Therefore, if OPC corrections are made based on the location of the simulation sites as originally positioned, the most optimum edge correction will likely not be achieved.

To achieve improved OPC corrections, it is desirable to place the simulation sites and/or use varying numbers of simulation sites at positions closer to where the EPE of an edge fragment is greatest along the length of an edge fragment.

SUMMARY OF THE INVENTION

To improve a resolution enhancement technique such as optical and process correction (OPC) of features to be created with a photolithographic process, the present invention divides layout features into a number of edge fragments. Simulation sites are positioned on one or more of the edge fragments in order to perform an initial calculation of image intensity. One or more of the simulation sites are then moved to be closer to a point of greater edge placement error (EPE) for an edge fragment. In one embodiment of the invention, one or more of the initially placed simulation sites are repositioned based on an image intensity gradient vector angle that is calculated at the simulation sites.

In another embodiment, additional simulation sites are positioned at locations on an edge fragment where the image intensity gradient vector indicates a curve in the image intensity along the edge fragment. In yet another embodiment of the invention, additional sample points are added to a simulation site where image intensities are calculated. In yet another embodiment, additional fragmentation end points are added or removed in accordance with the estimated image intensity gradient vectors. Image intensity calculations or EPEs that are calculated from the image intensities at the additional simulation sites or the additional sample points, are used to determine a desired OPC correction for the edge fragments.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:

FIGS. 1A-1C illustrate a conventional method of fragmenting a polygon into a number of edge fragments prior to performing OPC;

FIG. 2 illustrates a conventional method of positioning simulation sites on edge fragments;

FIG. 3A illustrates a fragmented polygon having an initial simulation site placement in accordance with one embodiment of the present invention;

FIG. 3B illustrates a number of image intensity gradient vectors that are computed at the simulation sites shown in FIG. 3A in accordance with an embodiment of the present invention;

FIG. 3C illustrates a fragmented polygon having a revised simulation site placement in accordance with one embodiment of the present invention;

FIG. 3D illustrates one method of determining where a simulation site should be repositioned based on a computed image intensity gradient vector in accordance with one embodiment of the present invention;

FIG. 4A illustrates a fragmented polygon having an initial simulation site placement in accordance with another embodiment of the invention;

FIG. 4B illustrates a number of image intensity gradient vectors that are computed at several of the initial simulation site placements;

FIG. 4C illustrates a fragmented polygon having additional simulation sites added to edge fragments in accordance with the computed image intensity gradient vectors according to another embodiment of the present invention;

FIG. 5 illustrates a simulation site having additional sample points added in accordance with another embodiment of the present invention;

FIG. 6 illustrates a grid of sample points at which simulations are performed for fragmenting a feature in accordance with another embodiment of the invention;

FIG. 7 illustrates a grid of sample points with a filter that eliminates sample points that are not in proximity to a feature edge; and

FIG. 8 illustrates one possible computing environment for performing the embodiments of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

To improve the optical and process correction of features to be created by a photolithographic process, the present invention uses a better placement of simulation sites and/or the addition of simulation sites, sample points or fragmentation end points to an edge fragment. Although the invention is primarily used in the creation of integrated circuits, it will be appreciated that the invention could be used with any feature to be created with a photolithographic process including Micro-Electrical-Mechanical Systems (MEMs), recording heads for disk drives, etc.

FIG. 3A illustrates a polygon 50 that defines a feature to be created on a wafer with a photolithographic process. The polygon 50 includes a number of fragmentation end points 52 that divide the perimeter of the polygon into a number of edge fragments. Associated with one or more of the edge fragments are simulation sites 54 a, 54 b, 54 c . . . 54 n, at which a number of the image intensity calculations are made. From the image intensity calculations, a determination can be made of the expected EPE for the corresponding edge fragment.

In one embodiment of the invention, the placement of one or more of the simulation sites 54 a-54 n is modified from their initial placement in order to improve the accuracy of the OPC corrections made to the edge fragments. As shown in FIG. 3B, a number of image intensity gradient vectors 56 are calculated at one or more of the initial simulation sites 54. In one embodiment of the invention, the image intensity gradient vectors 56 define the orientation of the image slope at the simulation site versus the orientation of the edge fragment.

As will be understood by those skilled in the art, each simulation site 54 includes a pattern of sample points at which the image intensity is simulated. The points generally form a cross with sample points oriented parallel to the edge fragment and sample points oriented perpendicular to the edge fragment. One method of calculating the image intensity gradient vector 56 is to estimate the image intensity at the sample points on either side of a center sample point and in a direction parallel to the edge fragment. Estimates of the image intensity on either side of the center sample point and in a direction perpendicular to the edge fragment are also made. From these estimates, a pair of vectors are computed and are mathematically combined in a head to tail fashion to compute the magnitude and direction of the image intensity at the area of the simulation site. The image intensity gradient vector 56 is indicative of the expected curvature of image intensity near the edge fragment.

Once the image intensity gradient vectors have been calculated, the results may be stored for the corresponding simulation sites with a tag or other identifier. Next, one or more of the simulation sites 54 are repositioned to be closer to a point of greater image intensity curvature for the edge fragment. As shown in FIG. 3C, a simulation site 54 c is moved to a position 54 c′, a simulation site 54 e is moved to a position 54 e′, and a simulation site 54 j is moved to position 54 j′, etc. With the simulation sites moved, a more accurate determination can be made how the edge fragments should be OPC corrected in order to produce better image fidelity.

FIG. 3D illustrates one example of moving a simulation site in accordance with the calculated image intensity gradient vector. At a simulation site 60, an image intensity gradient vector 62 is calculated to be oriented five degrees or less towards an adjacent fragmentation end point 64. Therefore, in one embodiment of the invention, the simulation site 60 is moved in the direction of the gradient to a position 60A that is 70% of the way between the original location of the simulation site 60 and the adjacent fragmentation end point 64. Similarly, at a simulation site 66, an image intensity gradient vector 68 is calculated to be oriented at an angle of greater than five degrees towards an adjacent fragmentation end point 64. Therefore, in one embodiment of the invention, the simulation site 66 is moved in the direction of the gradient, 100% of the way towards the adjacent fragmentation end point 64.

Although the example described above moves the simulation site in the direction of the gradient either 70% or 100% of the way towards an adjacent fragmentation end point, it will be appreciated that other distances could be used based on other magnitude and/or angle thresholds of the image intensity gradient vector. For example, if an image intensity gradient vector had an angle of less than 2°, no movement of the simulation site may be performed.

FIGS. 4A-4C illustrate another embodiment of the present invention. In this embodiment, a polygon 80 (FIG. 4A) is divided into a series of edge fragments using a number of fragmentation end points 82 that are positioned around the perimeter of the polygon 80. An initial placement of simulation sites 84 is made at which the image intensity is to be calculated. As shown in FIG. 4B, image intensity gradient vectors 86 are calculated at one or more of the simulation sites 84. For example, image intensity gradient vectors 86 a, 86 b, 86 c are calculated at simulation sites 84 a, 84 b, and 84 c respectively. If the image intensity gradient vector exceeds some predefined angle or magnitude, then one or more additional simulation sites 84 a 1 and 84 a 2 are added to the edge fragment on either side of the simulation site 84 a (FIG. 4C). Similarly, additional simulation sites 84 b 1, 84 b 2, and 84 c 1, 84 c 2 are added adjacent the simulation sites 84 b, 84 c. In one embodiment, the additional simulation sites are positioned on either side of the original simulation site. However, other placements may be used.

As an alternative to adding additional simulation sites to an edge fragment, each simulation site may have additional sample points added if the image intensity gradient vector exceeds a predefined angle or magnitude. As shown in FIG. 5, a polygon 100 representing a feature to be created by a photolithographic process includes a simulation site 102 having a number of sample points 102 a, 102 b, 102 c, etc., that are oriented in a direction perpendicular to the orientation of a corresponding edge fragment. In addition, the simulation site 102 includes a number of sample points 102 i, 102 j, 102 k, etc., that are oriented in a direction parallel with the edge fragment of the polygon.

A graph of the image intensity can be computed for the parallel and perpendicular sample points. For example, a graph 106 plots the changing image intensity as the sample points 102 a, 102 b, 102 c get closer towards the edge of the polygon. A graph 108 plots the image intensity at the sample points along the edge fragment of the polygon. If the image intensity along the edge fragment had little or no curvature, the graph 108 should be relatively flat. However, if the graph 108 has a curve, the image intensity is likely not consistent along the length of the edge fragment. Therefore, in one embodiment of the invention, additional sample points 110 a, 110 b, 110 c, etc., and 112 a, 112 b, 112 c, etc., can be added to the simulation site 102 if the image intensity varies by more than a predetermined amount along the length of the simulation site. In one embodiment, the additional sample points 110, 112, are oriented in a direction perpendicular to the length of the edge fragment. The image intensities can be calculated at each of the new additional sample points 110, 112, and the information used to calculate how the edge should be moved during OPC.

Once the placement of the simulation sites has been determined, or additional simulation sites and/or sample points added, an expected edge placement error (EPE) is determined for the edge fragments. The EPE is used to determine how the edge fragment should be OPC corrected, if at all. If the edge fragment includes more than one simulation site, a decision must be made regarding which image intensity data should be used in correcting the position of the edge fragment during OPC. For example, in one embodiment, expected EPEs are calculated at each simulation site or along each set of sample points on the edge fragment. The maximum EPE is then used in the OPC correction of the edge fragment. Alternatively, the minimum EPE for the edge fragment could be used or the average or some other mathematical combination of the EPEs could be used to determine how much, and in which direction, the edge fragment should be moved to improve image fidelity.

In yet another embodiment, the image intensity or EPE of an edge fragment may also be computed at each of the simulation sites/sample points assuming differing process conditions, such as illumination intensity, illumination pattern, focus, polarization, partial coherence settings, long range flare, etc. The image intensities or EPEs computed under each of the different process conditions are used alone or in combination to determine the OPC correction and/or fragmentation of an edge fragment.

Although the disclosed embodiment of the invention calculates an expected EPE for each simulation site and uses the EPE data to determine an OPC correction for an edge, it will be appreciated that it is not necessary to calculate an EPE at each simulation site. Rather, the image intensity data computed at each simulation site or set of sample points can be used alone or in combination to determine the OPC correction of the edge fragment. In addition, the adjustment of the simulation sites and/or sample points may occur a single time or multiple times during an OPC correction process, such that each iteration adjusts the location or number of one or more simulation sites and/or the number of sample points.

Although the embodiments of the invention described above use the calculated image intensity gradient vector to adjust the position of a simulation site, to add simulation sites to an edge fragment or to add sample points to simulation sites, it will be appreciated that the calculated image intensity gradient vectors can also be used to adjust the fragmentation of the polygons. For example, in areas where the image intensity gradient vector indicates a curving image intensity, additional fragmentation end points may be added. Conversely, where the calculated image intensity gradients indicate little curvature in the intensity gradient, fragmentation end points can be removed. In another embodiment, fragmentation end points can be added where the contour of an estimated image intensity of a designated value such as that required to properly expose a wafer, crosses an edge fragment. This designated value may be determined by a constant exposure threshold or calculated using a lithographic process model. The crossing points may be determined by interpolating the calculated image intensities that are estimated for neighboring simulation sites. Increasing the number of fragmentation end points generally improves pattern fidelity by allowing finer OPC adjustments but requires increased processing time. Removing fragmentation end points improves processing time at a cost of decreased OPC resolution. These steps can be repeated iteratively to optimize each step of the OPC procedure as it executes.

After refragmentation, simulation sites are added to the newly created edge fragments. In one embodiment, simulation sites are initially placed with a rule such as placing the site at the center of each edge fragment or according to the position of neighboring features, etc.

The initial placement can then be revised by calculation of the image intensity gradient vectors at the simulation sites and repositioning the simulation sites, adding more sites, or adding sample points to existing simulation sites as described above. The process can be repeated in an iterative manner. Furthermore, simulation sites associated with the edge fragments that are unchanged may be adjusted as a result of adding or removing fragmentation end points.

In yet another embodiment, the initial fragmentation and simulation site selection can be based on simulations calculated on a fixed grid of sample points regardless of the layout under consideration. For example, FIG. 6 shows a uniform grid 130 of sample points 132 at which estimates of image intensity are calculated regardless of the position of a feature 134 in a layout. Alternatively, as shown in FIG. 7, the uniform grid 130 may include a geometric filter 136 to eliminate sample points 132 that are not near the boundaries of the feature 134.

Once image intensity estimates have been made at each of the sample points 132, the feature 134 is fragmented to form a series of edge fragments that are OPC corrected. The image intensity calculations at the sample points 132 can determine the proper location of the fragmentation end points. Fragmentation end points can be placed at positions that are the closest to a sample point 132 where the image intensity has the desired value. Alternatively, the image intensity values can be interpolated to determine where the image intensity threshold crosses an edge of the feature and therefore where the fragmentation end points should be located.

In some instances, the position of the one or more sample points 132 associated with an edge fragment may be moved in accordance with an image intensity gradient vector as described above. One or more of the sample points 132 is associated with or mapped to each edge fragment for OPC purposes. The mapping may be made with a rule such as selecting the closest sample points next to an edge fragment or selecting the sample point with the least desirable image intensity that is near the center of the edge fragment. Alternatively, more complex algorithms may be used. The mapping of a sample point to an edge fragment may be static or dynamic during OPC iterations, etc.

FIG. 8 illustrates one possible computing environment for performing the present invention. A computer system 140 includes one or more processing units that perform a set of instructions that are stored on a computer readable media 142 or received embedded in a communication signal on a communication link to perform the methods of the present invention. An initial layout is stored in a conventional file format such as GDS-II, or an equivalent, on a database 142, computer readable media such as a CD, DVD, tape drive, etc., or is received over a communication link. The computer system 140 analyzes the layout to adjust the position of the simulation sites and/or adjusts the number of simulation sites/sample points or fragmentation end points in order to produce OPC corrected layout data in accordance with the embodiments of the invention as described above. The OPC corrected data is stored in a memory, on a computer readable media or in a database to be accessed by a mask writing tool (not shown) in order to produce one or more photolithographic masks or reticles used in a photolithographic process.

In an alternative embodiment of the invention, all or a portion of the initial layout can be transmitted to a remote computer system 160 that performs the fragmentation and simulation site selection/modification or re-fragmentation in accordance with the present invention. The remote computer system 160 may be in the same country as the computer system 140 or may be in a different country. The processed layout file or the OPC corrected layout data that is computed from the transmitted layout file is then transmitted to the computer system 140, or directly to the mask writing tool, via a wired or wireless communication link 162, such as the Internet, for use in creating photolithographic masks or reticles.

It will be appreciated that the relationship between fragmentation points and simulation sites and sample points can be complex. The techniques used in Matrix OPC, the subject of a previous U.S. patent application Ser. No. 10/387,224, hereby incorporated by reference, may also be applied to manage these relationships. While the disclosed embodiments have been primarily directed to performing OPC on the layout description, it will be appreciated that the present invention is also useful with other resolution enhancement techniques including: generating phase-shifting mask layouts, compensating for off-axis illumination systems, compensating for polarization effects and techniques for compensating for multiple exposures.

While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the scope of the invention. It is therefore intended that the scope of the invention be determined from the following claims and equivalents thereof. 

1.-28. (canceled)
 29. A method of preparing a layout data file defining a number of features to be created via a photolithographic process for the application of a resolution enhancement technique, comprising: reading at least a portion of a layout data file defining features to be created; estimating a number of image intensities at sample points around each feature; fragmenting each feature into a number of edge fragments based on the estimated image intensities; and associating an image intensity at a sample point with an edge fragment for use in the application of a resolution enhancement technique on the edge fragment.
 30. The method of claim 29, wherein the feature is fragmented into edge fragments having fragmentation end points that are positioned at locations where a contour of an image intensity of designated value intersects with a feature edge.
 31. The method of claim 30, further comprising: calculating an image intensity gradient vector at one or more simulation sites; and moving the simulation sites in accordance with the image intensity gradient vectors.
 32. The method of claim 29, further comprising: calculating an image intensity gradient vector at one or more simulation sites; and removing edge fragments from a feature where adjacent image intensity gradient vectors are substantially the same.
 33. The method of claim 29, further comprising: calculating an image intensity gradient vector at one or more simulation sites; and adding edge fragments to a feature where adjacent image intensity gradient vectors vary by more than a predetermined amount.
 34. The method of claim 33, wherein the simulation sites are in a uniform geometric pattern.
 35. The method of claim 34, wherein the uniform geometric pattern has a filter to eliminate simulation sites that are not adjacent a border of the feature.
 36. A computer readable storage device or memory storing data defining a layout for a number of features to be created via a photolithographic process, wherein said data is created by: reading at least a portion of a layout data file defining features to be created; estimating a number of image intensities at sample points around each feature; fragmenting each feature into a number of edge fragments based on the estimated image intensities; and associating an image intensity at a sample point with a first edge fragment for use in the application of a resolution enhancement technique for the first edge fragment.
 37. A method of preparing a layout data file defining features to be created via a photolithographic process, comprising: transmitting at least a portion of the layout data file to a remote computer system for processing by: reading at least a portion of a layout data file defining features to be created; estimating a number of image intensities around each feature; fragmenting each feature into a number of edge fragments based on the estimated image intensities; and assigning a simulation site to each edge fragment for use in the application of a resolution enhancement technique for the edge fragment.
 38. A system, comprising: at least one processor and memory attached thereto; and a computer readable storage device or memory storing computer-readable instructions that when executed by the processor, cause the processor to perform a method, the instructions comprising: instructions to estimate a number of image intensities at sample points around each of a number of features read from a layout data file, instructions to fragment each feature into a number of edge fragments based on the estimated image intensities, and instructions to associate an image intensity at a sample point with an edge fragment for use in the application of a resolution enhancement technique on the edge fragment.
 39. The system of claim 38, wherein the instructions further comprise: instructions to apply the resolution enhancement technique to the features based on the associated image intensities; and instructions to transmit or store data representing layout enhanced by the applied the resolution enhancement technique in a computer-readable storage device or memory.
 40. The system of claim 38, wherein at least one feature is fragmented into edge fragments having fragmentation end points that are positioned at locations where a contour of an image intensity of designated value intersects with a feature edge.
 41. The system of claim 38, wherein the instructions further comprise: instructions to calculate an image intensity gradient vector at one or more simulation sites; and instructions to move the simulation sites in accordance with the image intensity gradient vectors.
 42. The system of claim 38, wherein the instructions further comprise: instructions to calculate an image intensity gradient vector at one or more simulation sites; and instructions to remove edge fragments from a feature where adjacent image intensity gradient vectors are substantially the same.
 43. The system of claim 38, wherein the instructions further comprise: instructions to calculate an image intensity gradient vector at one or more simulation sites; and instructions to add edge fragments to a feature where adjacent image intensity gradient vectors vary by more than a predetermined amount.
 44. The system of claim 43, wherein the simulation sites are in a uniform geometric pattern.
 45. The system of claim 44, wherein the uniform geometric pattern has a filter to eliminate simulation sites that are not adjacent a border of the feature. 